BBT tips and suggestions

Hello! So I've been tracking my ovulation through CM and opk, but CM is inconsistent and opk may not be enough. I've decided to use bbt, but I don't think I'm doing it right despite reading instructions on the internet. My problem is I get a lot of difference each day. Variance is always more than .5 to 1 and this has been the case even though I'm still so far from ovulation. Thermometer is working fine as I have compared it with my other thermometer. I'm also doing it orally.

Any tips or suggestions how to do this the right way? Thank you so much!